1. Field of the Invention
The present invention relates to a noise removal device and a noise removal method, and more specifically, to a noise removal device and a noise removal method for removing noise generated in a radio receiver.
2. Description of the Related Art
In an in-vehicle radio receiver, when pulse noise such as ignition noise generated by the engine of a vehicle gets into an in-vehicle radio receiver, the audio quality of a radio signal received by the radio receiver is deteriorated. For this reason, the radio receiver is provided with a noise removal device for removing such pulse noise from a reception signal.
In the noise removal device, correction is performed on an audio signal in which noise is generated, by using a scheme such as a previous-value hold scheme or an interpolation scheme between two known points (linear interpolation scheme). In the linear interpolation scheme, the point at which noise is detected is set as a start point, and the point after a predetermined correction period from the start point is set as an end point. Then, the audio signal between these two points is interpolated. In the linear interpolation scheme, the correction period is generally determined in advance so as to facilitate processing for calculating a correction amount. However, when the period in which noise is generated is long, the noise cannot be reduced satisfactorily with the linear interpolation scheme. This is because of the following reasons.
Even though pulse noise is generated for a long period, a signal for the predetermined correction period (hereinafter, simply referred to as a correction period signal) may be shorter than the actual noise generation period. FIGS. 6A and 6B show examples of noise correction performed by using the linear interpolation scheme and by using the previous-value hold scheme, respectively, when the correction period signal is shorter than the pulse-noise generation period. As shown in FIG. 6A, when the linear interpolation is performed on an audio signal including pulse noise of a period t1 by using the correction period signal shorter than the period t1, the pulse noise is included in a value at the end point of the correction period used for the linear interpolation. As a result, not only satisfactory correction fails to be performed, but a large correction error might even result. A possible way to avoid this is to perform the previous-value hold scheme, as shown in FIG. 6B, on the audio signal so that the correction error can be made small.
To solve the problem caused by a mechanism employing only the linear interpolation scheme, Japanese Patent Application Publication No. 2004-056173 (referred to as Patent Document 1 below) discloses a technique of switching between the linear interpolation scheme and the previous-value hold scheme. FIG. 7 is a block diagram of a conventional noise removal device disclosed in Patent Document 1. The conventional noise removal device includes an FM demodulation circuit 1, a stereo demodulation circuit 5, a previous-value hold scheme correction circuit 14, a noise detection circuit 21, a correction scheme determination circuit 22, a linear interpolation scheme correction circuit 23, and a switch circuit 24.
In FIG. 7, the noise detection circuit 21 detects a noise generation period from an FM demodulation signal outputted from the FM demodulation circuit 1, and outputs a correction period signal. Based on the correction period signal and the FM demodulation signal, the correction scheme determination circuit 22 determines a correction scheme. When the correction period signal is determined as shorter than the actual noise generation period, the switch circuit 24 switches output from the linear interpolation scheme correction circuit to output from the previous-value scheme correction circuit 14. Then, the output from the previous scheme correction circuit 14 is converted into an audio signal by the stereo demodulation circuit 5.
As mentioned above, when a correction period signal is shorter than a pulse-noise generation period, the previous-value hold scheme causes less adverse effects on correction than the linear interpolation scheme. For this reason, in Patent Document 1, the correction scheme determination circuit 22 determines whether the correction period signal is shorter or longer than the pulse-noise generation period. Then, the correction scheme determination circuit 22 outputs a correction scheme selection signal selecting the previous-value hold scheme when the correction period signal is shorter, and outputs a correction scheme selection signal selecting the linear interpolation scheme when the correction period signal is longer.
Using FIGS. 8A to 8D, a detailed description will be given of how the correction scheme determination circuit 22 performs determination. When, as shown in FIG. 8C, a correction period signal is shorter than a pulse-noise generation period in an FM demodulation signal shown in FIG. 8A, pulse noise exists in the FM demodulation signal at an end point of the correction period signal. This causes a large difference between values of the FM demodulation signal at a start point and the end point of the correction period (|S−E1|). In contrast, when the correction period signal properly matches the pulse-noise generation period as shown in FIG. 8D, a difference between values of the FM demodulation signal at the start point and the end point of the correction period (|S−E2|) is smaller than an average amplitude level of the FM demodulation signal. Based on this, the correction scheme determination circuit 22 determines that the correction period signal is shorter than the pulse-noise generation period when a difference between the start point and the end point (|S−E|) is larger than an average amplitude P of the FM demodulation signal shown in FIG. 8B.
The conventional noise removal device is designed based on the assumption that the main noise source of the vehicle is ignition noise having a pulse noise generation interval of one to several ms or more. For this reason, it is sometimes difficult for the noise removal device of Patent Document 1 to handle high frequency pulse noise having short cycles, such as noise generated from a door mirror drive motor or a hazard flasher. Specific cases are described below.
For example, consider a case of performing noise correction processing on a signal including pulse noise having small amplitude as shown in FIG. 9A. In this case, even when the correction period signal shown in FIG. 9C is shorter than the pulse-noise generation period, a difference between the start point and the end point of a correction period |S−E| is smaller than the average amplitude level P of the FM demodulation signal shown in FIG. 9B. Consequently, the correction scheme determination circuit 22 wrongly determines that the correction period signal is longer than the pulse-noise generation period, and therefore selects the linear interpolation scheme. As a result, the FM demodulation signal after the noise correction processing has a waveform in which the start point and the end point of the correction period for the noise correction processing are connected. Accordingly, a large correction error is caused. Under the policy of selecting the interpolation scheme based on the average amplitude of the FM demodulation signal as in the above way, the linear interpolation scheme might be wrongly selected if the amplitude of the noise is small, even when the correction period signal is shorter than the pulse-noise generation period.
In addition, a description will be given of a case of performing the noise correction processing on an FM demodulation signal including pulse noise events successively at a short interval as shown in FIG. 10A. FIG. 10B is an enlarged waveform diagram of the portion XB in FIG. 10A. Further, FIGS. 10C to 10E show a waveform diagram of a noise detection signal, of a correction period signal, and of a noise period detection signal, respectively.
For example, when the interval between two pulse noise events is nearly equal to (the same as or shorter than) a preset correction period as shown in FIGS. 10B to 10D, the detected noise generation period indicates as if the noise were already finished at the end position of the noise correction processing, as shown in FIG. 10E. Accordingly, the correction scheme determination circuit 22 wrongly determines that the correction period signal is longer than the pulse-noise generation period, and therefore selects the linear interpolation scheme. As a result, the delayed FM demodulation signal shown in FIG. 10F has a waveform in which the start point and the end point of the correction period are connected as shown in FIG. 10G. Consequently, a large correction error is caused. When the interpolation scheme is selected based on the state of the end point of the correction period, proper correction cannot be performed for noise events included successively at a short interval.
Furthermore, the pulse-noise generation period and the correction period are compared; therefore, as shown in FIG. 11A, a noise event after the end point of the correction period cannot be corrected. Patent Document 1 discloses a method for revising the correction period by applying the detection result of the noise generation period to the correction period as shown in FIG. 11B. However, a proper correction period cannot be obtained with this revision method when the above-described correction period signal is shorter than the pulse-noise generation period, or when the signal includes pulse noise events successively at a short interval. As described, in some cases, correction cannot be performed on a noise event that occurs after the correction period.